certbotで使い捨てSSL証明書を発行する

参考: https://qiita.com/ysogabe/items/1ffc1d381b5770ab57a8

```
$ docker run -it --rm --entrypoint /bin/sh certbot/certbot:v0.23.0
$ certbot certonly \
  --manual \
  --preferred-challenges dns \
  -d *.<証明書を取得したいドメイン> \
  --server https://acme-v02.api.letsencrypt.org/directory \
  --register-unsafely-without-email
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Registering without email!
Starting new HTTPS connection (1): acme-v02.api.letsencrypt.org

-------------------------------------------------------------------------------
Please read the Terms of Service at
https://letsencrypt.org/documents/LE-SA-v1.2-November-15-2017.pdf. You must
agree in order to register with the ACME server at
https://acme-v02.api.letsencrypt.org/directory
-------------------------------------------------------------------------------
(A)gree/(C)ancel: A
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for <証明書を取得したいドメイン>

-------------------------------------------------------------------------------
NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you're running certbot in manual mode on a machine that is not
your server, please ensure you're okay with that.

Are you OK with your IP being logged?
-------------------------------------------------------------------------------
(Y)es/(N)o: Y

-------------------------------------------------------------------------------
Please deploy a DNS TXT record under the name
_acme-challenge.<証明書を取得したいドメイン> with the following value:

z6dhYCEe2KD6e0bK5IhdF9aT7ReKMOagCiD8oVBkM2c

Before continuing, verify the record is deployed.
-------------------------------------------------------------------------------
Press Enter to Continue
```

ここまで表示されたら Enter を押さずに書いてある通りに DNS のTXTレコードの設定を行う
設定したら Enter を押すと以下の様にSSL証明書が発行される

```
Waiting for verification...
Cleaning up challenges

IMPORTANT NOTES:
 - Congratulations! Your certificate and chain have been saved at:
   /etc/letsencrypt/live/<証明書を取得したいドメイン>/fullchain.pem
   Your key file has been saved at:
   /etc/letsencrypt/live/<証明書を取得したいドメイン>/privkey.pem
   Your cert will expire on 2019-11-03. To obtain a new or tweaked
   version of this certificate in the future, simply run certbot
   again. To non-interactively renew *all* of your certificates, run
   "certbot renew"
 - Your account credentials have been saved in your Certbot
   configuration directory at /etc/letsencrypt. You should make a
   secure backup of this folder now. This configuration directory will
   also contain certificates and private keys obtained by Certbot so
   making regular backups of this folder is ideal.
 - If you like Certbot, please consider supporting our work by:

   Donating to ISRG / Let's Encrypt:   https://letsencrypt.org/donate
   Donating to EFF:                    https://eff.org/donate-le
```


これで完了
生成された証明書は `IMPORTANT NOTES:` の項目にあるように

証明書: /etc/letsencrypt/live/<証明書を取得したいドメイン>/fullchain.pem
秘密鍵: /etc/letsencrypt/live/<証明書を取得したいドメイン>/privkey.pem

に生成されるのでそれを保存する

今回は使い捨てで更新の予定がなかったので --register-unsafely-without-email オプションをつけたが、このオプションを外せば email を質問されるので更新のメールが送られるようになるらしい